I think you're wrong. Itch is a great site, especially for new developers. It's easy, especially compared to the alternatives. We're lucky to have it, and I think they deserve their share. It's very rare and unbelievably nice that they even allow us to choose the amount.
Yes! But they should charge extra for adult content. The charge is only on paid sums anyways. But I know taht certain individuals press report on content that they deem bad, so I claim that adult content will have more maintenance. Higher share is justfifyable.
As for payments, it is about payout to the devs. You can have payout in paypal if you get approvment of pp to sell adult content. I do not know requirement, but probably it includes being in a country that allows adult stuff.
Itch should reasearch and consider methods of real hardcore stuff payments. Or how do all those web caming people collect their money? They face similar situation as (adult) indie dev.
From user side, I can pay for adult stuff with paypal no problem. Many things are on patreon. It is not understandable, why I can pay to the same creator for the same content on patreon with paypal but not on itch. So how does patreon pay out to devs?